The invention relates to a manometer device for continuous monitoring of the air pressure in vehicle tires, wherein a membrane and a magnet movable therewith are inserted in a manometric capsule consisting of a base with a cover attachable fluidtightly thereon, which capsule responds to variations in the air pressure of tires and transmits them to a terminal.
Tire-monitoring appliances in which a magnetc is moved or the magnetic field is modified in another manner by membranes or manometric capsules as a function of the prevailing tire pressure have been proposed in numerous forms of construction.
An alarm system for low tire pressure is known from German Offenlegungsschrift No. 2,556,047, wherein a magnet is provided with a first and second pole surfaces in order to form a closed magnetic flux in the normal position of the magnet. The device exhibits a low-pressure air switch in a chamber system which is provided with a plurality of valves and an air bellows which supplies the reference pressure. The low-pressure air switch is attached to the rim of the vehicle tire and extends into the tire. When the tire pressure falls, a piston sealed by an O-ring is activated, which releases the bellows movement. The magnet is then moved out of the normal position into an extended position. The external magnetic field thereby generated is sensed by a stationary sensing device in order to generate a signal.
A similar monitoring system with magnetic signalling, which is installed in the interior of the rim and therefore in the air space of the vehicle tire, is described in German Offenlegungsschrift No. 2,848,942. It is therefore necessary for the appliance to be installed and adjusted by charging with helium gas before the tire is fitted. This monitoring system also consists of numerous components, such as an expansible bellows and a magnet unit associated therewith with permanent magnet and pole piece, a plate made of ferromagnetic material. The fact that the appliance is no longer accessible after the tire has been fitted is particularly disadvantageous. Furthermore, the magnet is moved in the radial direction toward the vehicle wheel.
A tire pressure gauge with a magnetic transmission device for the measured value of vehicle tires is known from German Offenlegungsschrift No. 3,029,246. This pressure gauge consists of a manometric capsule with a corrugated stainless steel membrane, upon which the prevailing tire pressure acts through a bore in the rim. The corrugated membrane carries a magnet which is moved radially to the wheel as a function of the tire pressure. In addition, a reference magnet is provided which revolves with the rotation of the wheel but which does not execute a radial movement. During each wheel revolution, both magnets are carried past an induction coil and the induced voltages are processed electrically so that they can be compared. Voltage differences which appear indicate a drop in pressure in monitored tires.
All of the manometric systems which have hitherto become known for continuous monitoring of the air pressure in vehicle tires are extremely complicated in construction and consist of a large number of individual components. Furthermore, in the case of the majority of systems, the rim of the vehicle wheel has to be provided with a bore, an undesirable feature from an operational standpoint. Stringent demands are also imposed upon the fluidtightness of the transducer housings because the smallest drop in pressure in the reference chamber can lead to faulty indications. The majority of transducers are equipped with springs and corresponding adjusting screws, and as a result, the temperature sensitivity of such monitoring appliances is very high. This creates the necessity for additional components to compensate for temperature fluctuations. The monitoring systems proposed are described with transducers which are fitted to the rim so that the movement of the elements supporting the magnet occurs radially to the vehicle wheel. Due to this placement on the rim, extremely high centrifugal forces come into play, at least in some cases, which are a function of the rim diameter and of the speed of travel, and therefore, such forces fluctuate a great deal.